Geochemical fingerprinting of volcanic tephra layers for the correlation of coal layers in the Paleocene Tullock formation, USA

Geochemical fingerprinting of volcanic tephra layers for the correlation of coal layers in the Paleocene Tullock formation, USA

Rost, E.

Molengraaff Fonds

2014

The Paleocene Tullock formation, located in the North American Western Interior Basin, contains widely distributed coal layer deposits which display various cyclic patterns. The formation of the cyclicity is generally attributed to regional to local tectonic variation processes. However, there are several examples of sediment deposits that are interpreted as allocyclic sediments, influenced by Milankovitch Cyclicity in the North American Western Interior Basin (Elder et al., 1994; Meyers et al., 2012; Sageman et al., 2014). This could also be the case for the early Paleocene Tullock formation, in which several sedimentary cycles could be counted, interpreted as precession (~26 k.y.) and eccentricity cycles (~100 k.y.). However, to confirm this theory a long distance correlation of the sedimentary layers is necessary. Furthermore, to enable extensive correlation key isochrons are required. Suitable candidates that could act as a reliable key isochron are volcanic ash layers, so-called “tephra” layers. The tephra layers can be correlated based on their geochemical properties. This method is called the method of tephrochronology (Lowe, 2010). To obtain reliable correlation, geochemical fingerprints of the tephra layer samples were determined, through major (EMP) and trace element (ICP-MS) analyses. To place the geochemical analyses in a relative time framework paleomagnetic samples were obtained. The geochemical measurements provide information about the geochemical properties of the tephra layers and the possible source(s) of the tephra layers. The paleomagnetic measurements provide information about the duration of C29r, the stratigraphic position of C29r/C29n reversal and the sedimentation rate of C29r. The results of this research project show that the identification of geochemical fingerprints of coal layers or single tephra layers is not possible with the major and trace element analyses and the melt variation models produced. However, there are indications that major element analyses of a more extensive biotite sample batch could provide a method of correlation of tephra layers. Furthermore, all geochemical methods applied here indicate that a general geochemical trend was present within the studied section of Tullock formation, the tephra layers become increasingly more evolved from the lower to the upper part of the section. The melt variation models show that there are at least three geochemically distinct magmatic sources for the tephra layers of the studied section. Additionally, the outliers of the major and trace element variation diagrams indicate that it is very likely that the number of sources is higher. The magmatic evolution scenario that best fits the trend the main source group is that the tephra layers originate from a geochemically differentiated magma chamber. The physical location of this magma chamber cannot be determined from the results obtained for this research. However, since we are dealing with subduction related magmatism it can be assumed that the source is located at the Cordilleran Orogeny ~700 km to the west. A likely candidate would be the Late Cretaceous – Early Paleogene (~70-50 Ma) Bitterroot Lobe of the Idaho Batholith. This means, that with the information obtained from this project, influences of Orbital Climate forcing during deposition of the sediments from the Tullock formation, cannot be confirmed nor rejected. Hence, further research is necessary.

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Vrije Universiteit Amsterdam

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